The invention relates to a method of detecting pulsatory interference signals in an audio signal.
A method of this type is known, inter alia, from German Patent Specification 2.912.689 and is used, for example, in FM car radio receivers having a circuit for suppressing the pulsatory interference signals (IAC=Interference Absorption Circuit). In these receivers the received and demodulated multiplex signal is passed through a restoration circuit which is blocked during the occurrence of an interference pulse, which pulse is replaced, for example, by the signal level stored in a memory just before the occurrence of the interference pulse.
For detecting the interference pulses in the received multiplex signal the fact is utilized that the interference pulses comprise frequency components above the frequency band of the useful multiplex signal. For example, in FM stereo receivers the demodulated multiplex signal covers the frequency range up to approximately 60 kHz. By passing this signal through a high-pass filter having a cut-off frequency of approximately 60 kHz, the components of the useful multiplex signal are suppressed to a considerable extent. A signal mixture then remains which, in addition to noise, comprises pulsatory interference signals extending beyond the noise level, which interference signals can subsequently be detected by means of a threshold circuit.
However, the known method has a number of drawbacks which have gained greater significance in the course of time. The major drawbacks are:
1. Due to the increase of the number and strength of neighbouring transmitters, the frequency range above 60 kHz of a multiplex signal is increasingly disturbed so that the selection of the pulsatory interferences from the signals present in this frequency range is considerably worse. PA0 2. The increased IF amplification between the last intermediate-frequency filter and the FM detector produces a noise spectrum in said frequency range which is more unfavourable for a satisfactory detection of interference at a low field strength. PA0 3. The trend towards greater channel selectivity attenuates noise and neighbouring channel interference in said frequency range, but it attenuates to an even greater extent the components of the pulsatory interference signals in this range. Detection of these interference signals in accordance with the known method will therefore become substantially impossible.